Traditionally, methylene chloride has been used in paint strippers because of its high efficacy and low cost. Because methylene chloride is toxic, it is being replaced by less toxic, non-halogenated solvents such as N-methyl-2-pyrrolidone, dibasic esters, and other solvents.
The use of N-methyl-2-pyrrolidone (NMP) and dibasic esters in paint strippers, often in combination with aromatic solvents, is described, for example, in U.S. Pat. Nos. 4,120,810, 4,732,695, 4,749,510, and 4,780,235. These compositions are effective on a broad range of coatings, including some crosslinked coatings. Unfortunately, the aromatic solvents and dibasic esters are typically not water-miscible, so water rinsing is not feasible.
Water-rinsable compositions are taught in European Patent Application 0 294 041, in which diesters of C.sub.4 -C.sub.6 dibasic acids are combined with water and a thickener. European Patent Application 0 389 829 teaches a water-reducible formulation that incorporates NMP, diesters of C.sub.4 -C.sub.6 dibasic acids, glycol ethers, and a thickener. European Patent Application 0 355 763 teaches a water-reducible formulation that includes NMP, aromatic solvents, a glycol ether, an alkanolamine, and a thickener. The formulations described in all of these European applications are water rinsable. However, none of these is effective for stripping highly crosslinked coatings.
Activators, such as organic acids or organic bases, are sometimes used in paint strippers to enhance stripping ability. For example, U.S. Pat. No. 4,749,510 teaches to use formic acid to improve the performance of an NMP/aromatic solvent blend. International Application WO 88/06640 teaches that combinations of NMP or .gamma.-butyrolactone (GBL) with an organic acid and a glycol ether are effective for stripping uncured paints. Performance results of GBL-containing compositions are not provided.
Thus, in spite of the availability of water-rinsable coating removers, the need remains for water-rinsable compositions that are effective for stripping highly crosslinked coatings.
It is an object of this invention to provide aggressive coating removers that are free of methylene chloride, water rinsable, and capable of stripping highly crosslinked coatings such as two-part epoxies and polyurethanes. It is another object of the invention to provide coating removers with high flash points to minimize fire hazards. It is another object of the invention to provide low-toxicity, low-cost compositions having a low content of volatile organic compounds (VOC) to minimize potential pollution of the environment.